The present invention relates to the art of magnetic resonance imaging (MRI). It finds particular application in conjunction with open geometry magnet systems, and will be described with particular reference thereto. However, it is to be appreciated that the present invention is also amenable to other like applications where high-quality homogenous magnetic fields are desired.
One common type of MRI system has a patient receiving bore through which a main magnetic field is generated longitudinally by a surrounding series of annular magnet windings. A patient or subject is selectively translated axially along a horizontal central axis of the bore to have a region of interest positioned in an imaging volume for imaging. In these solenoid type annular bore systems, access to the patient for surgical and/or invasive procedures, physiological tests, equipment, and the like is limited and awkward. Moreover, such systems tend to be claustrophobic for some patients.
To provide for access and reduce the claustrophobic effect in patients, open or vertical field magnets have been developed. Open magnets typically include a ferrous flux return path in the form of a "C", "H", or four-poster arrangement. The flux return paths have an open gap or patient receiving region within which the patient or subject is disposed for imaging. Typically, two annular magnets are disposed on opposing sides of the gap to generate the main magnetic field or magnetic flux therethrough. Due to the difference in the susceptibility of the flux return path and the air in the gap, there tends to be non-uniformity and other magnetic flux errors in the gap. However, high image quality is dependent on having the main magnetic field in the imaging volume as homogeneous and perturbation free as possible. In order to generate a stronger, more uniform magnetic flux through the gap, ferrous pole pieces are typically positioned at the ends of the flux return path on either side of the gap or patient receiving region. In some cases, the pole pieces are shaped and contoured with features such as annular ridges and grooves, as appropriate, to generate a more uniform or homogeneous magnetic flux between the pole pieces. For example, see U.S. Pat. Nos. 5,436,607 and 5,162,768 which show contoured and/or shaped pole pieces for enhancing the magnetic field in the imaging volume.
Although the use of contoured and/or shaped pole pieces has certain advantages, there are trade-offs. In particular, with reference to the '607 patent, one trade-off is that the features are concentrated on the back sides of the poles so as not to reduce the patient gap. Poles with contour features on the front side thereof are considered advantageous over those having features on the back because contour features are generally more effective the closer they are to the imaging volume. At higher field strengths, to achieve the same level of effectiveness as at lower field strengths, the contoured features are more pronounced (i.e. the grooves are deeper, etc.). This sensitivity to position of pole shaping features allows some designs to be realized with poleface features that cannot be realized with shaping behind the pole.
The present invention contemplates a new and improved pole piece and MRI scanner employing the same which overcomes the above-referenced problems and others.